1. Field of the Invention
The present invention relates to an optical frequency converter that performs frequency conversion between light and a terahertz wave including an electromagnetic wave component in a frequency region ranging from a millimeter waveband to a terahertz waveband (30 GHz to 30 THz), and to a device equipped with the optical frequency converter. In particular, the present invention relates to an element that generates an electromagnetic wave including a Fourier component in the aforementioned frequency region by emitting a laser beam to a nonlinear optical crystal, or that detects the electromagnetic wave, and to a tomography device based on terahertz time-domain spectroscopy (THz-TDS) equipped with such an element.
2. Description of the Related Art
In the frequency region of a terahertz wave, many organic molecules of a biomaterial, a medicinal drug, an electronic material, or the like have an absorption peak deriving from the structure or the state thereof. Furthermore, terahertz waves have high transmissivity through materials such as paper, a ceramic material, resin, and fabric. In recent years, research and development of imaging and sensing technologies that utilize such characteristics of terahertz waves have been implemented. For example, terahertz waves are expected to be applied to fluoroscopic inspection devices, which are safe, in place of X-ray devices, or to in-line non-destructive inspection devices used in manufacturing processes. A widely used method of generating a terahertz wave is a method that uses a nonlinear optical crystal. Examples of a nonlinear optical crystal include LiNbOx (which will sometimes be referred to as “Lithium Niobate” or simply “LN” hereinafter), LiTaOx, NbTaOx, KTP, DAST, ZnTe, GaSe, GaP, and CdTe. Here the subscript “x” stands for a positive integer. A terahertz wave is generated by utilizing a second-order nonlinear phenomenon, and difference-frequency generation (DFG) based on input of two laser beams having different frequencies is known. In nonlinear crystal materials, DFG can occur where two laser beams generate another beam with the difference of the optical frequencies of the two laser beams. Moreover, monochromatic terahertz-wave generation based on an optical parametric process and terahertz-pulse generation based on optical rectification using a femtosecond pulsed laser beam are also known.
As a process for generating a terahertz wave from the aforementioned nonlinear optical crystal, electro-optical Cerenkov radiation has recently been receiving attention. Specifically, referring to FIG. 8, Cerenkov radiation is a phenomenon in which, when a propagation group velocity of a laser beam 2 acting as an excitation source is higher than a propagation phase velocity of a generated terahertz wave, a terahertz wave 1 is conically released like a shock wave. Based on a refractive-index ratio between the light and the terahertz wave within a medium (i.e., nonlinear optical crystal), a radiation angle θc of the terahertz wave is determined from the following expression:cos θc=vTHz/vg=ng/nTHz where vg and ng respectively denote a group velocity and a group refractive index of the excitation light, and vTHz and nTHz respectively denote a phase velocity and a refractive index of the terahertz wave. There has been a report with regard to generation of a high-intensity terahertz pulse based on optical rectification by making a femtosecond laser beam with a tilted wavefront enter LN by utilizing the Cerenkov radiation phenomenon. See, Hebling et al., “Generation of high-power terahertz pulses by tilted-pulse-front excitation and their application possibilities”, J. Opt. Soc. Am. B, Vol. 25, pp. B6-B19 (2008) (Document 1). Moreover, in order to eliminate the need for tilting the wavefront, there has also been a report with regard to the use of a slab waveguide having a thickness smaller than the wavelength of a generated terahertz wave so as to generate a monochromatic terahertz wave on the basis of the DFG method. See, Suizu et al., “Extremely frequency-widened terahertz wave generation using Cherenkov-type radiation”, Opt. Express, Vol. 17, pp. 6676-6681 (2009) (Document 2).
Because a terahertz wave is generated by excitation of a traveling wave in the examples in Document 1 and Document 2, extraction efficiency is increased by phase-matching in the radiation direction and combining the intensity of terahertz waves generated from different wave sources. Characteristic features of this radiation method include a capability to achieve relatively high efficiency and generate a high-intensity terahertz wave when a nonlinear optical crystal is used, and a capability to broaden the terahertz waveband by selecting a high-frequency side for the absorption in a terahertz region due to phonon resonance, which is a distinctive feature of crystals. These technologies allow for a broader generation band, as compared with when a terahertz wave is generated by a photoconductor, and also allow for a narrower pulse width when a terahertz pulse is generated by utilizing optical rectification. For example, in the case where these technologies are applied to a terahertz time-domain spectroscopy device, it is expected that the device performance can be enhanced.
However, with the technique discussed in Document 1, it may not be easy to efficiently converge and control the terahertz wave that is Cerenkov-radiated in a conical shape. For example, when the slab waveguide formed on a substrate disclosed in Document 2 is used, only a component above the substrate can substantially be used from the terahertz wave radiated in all directions. Therefore, there is a limit with regard to the enhancement of the conversion efficiency. Moreover, it is not easy to couple the excitation light to the slab waveguide, made of a nonlinear-optical-crystal thin film having a thickness of about several micrometers, with high efficiency, and because nonlinear polarization based on the excitation light is proportional to the square of the input electric field, there is room for an improvement in terms of efficiency.